The purpose of this project is to establish the locus and extent of retention of aerosol particles in the respiratory tracts of laboratory animals. Sub-gross anatomies of the respiratory tracts are being investigated with the aid of casts. Calculations are made regarding deposition of aerosols by impaction, settling and diffusion. Direct measurements of retention will be made by analysis of inhaled and exhaled air. Retentions are also estimated indirectly from toxicological effects of exposure to aerosols of various substances. The ultimate aim is to be able to interpret a given aerosol exposure for a laboratory animal in terms of effects to be expected in man.